In the course of various assays and analyses, sample material may be segmented into discrete plugs (or droplets) of fluid spatially separated from each other by a different type of fluid that is immiscible with the sample-containing plugs. For example, the sample plugs may be aqueous while the other fluid utilized to isolate the sample plugs may be organic. Known phase segmenting devices are capable of receiving flows of sample material and isolating material and forming a linear (one-dimensional) array of sample plugs alternately separated by non-sample plugs (or isolator plugs). If the bore of a conduit is small enough (e.g., a capillary tube or microfluidic channel), a linear, segmented sample array can be transported through the conduit to a desired destination, such as a detector or an analytical instrument, while preserving the alternating sequence of sample plugs and non-sample plugs.
In various applications, segmented sample arrays can provide a number of advantages. Each sample plug may serve as an individual medium for transporting the sample material contained therein, with limited or no loss of sample material to the surrounding immiscible phase. Moreover, each sample plug may serve as a self-contained reactor that is isolated from other sample plugs, thus reducing the risk of cross-contamination and enabling different reactions to be carried out in different sample plugs. Moreover, the use of sample plugs can minimize the total volume of sample, reagents, and other material required for a given experiment. Further, the large surface area to volume ratio of sample plugs enables rapid heat transfer (heating or cooling of the sample material).
Segmented sample arrays may potentially be utilized in a wide range of sample processing applications. Applications include, for example, liquid-liquid extraction (LLE) (see, e.g., Silvestre et al., Liquid-liquid extraction in flow analysis: A critical review, Analytica Chimica Acta, 652, 54-65 (2009)), post-chromatography column reaction chemistry (see, e.g., Nie et al., Capillary liquid chromatography fraction collection and postcolumn reaction using segmented flow microfluidics, J. Sep. Sci., 36, 3471-3477 (2013)), and a wide range of droplet-based microfluidic and lab-on-a-chip applications (see, e.g., Schneider et al., The Potential Impact of Droplet Microfluidics in Biology, Anal. Chem., 85, 3476-3482 (2013)).
Segmented flow systems may be interfaced to detectors and analytical instruments. Of recent interest is the injection of segmented sample arrays into electrospray ionization-mass spectrometry (ESI-MS) systems, as segmented sample arrays can potentially meet the sensitivity and time-scale requirements of ESI-MS and MS is a label-free analytical technique. Some prior interfaces have relied on the use of phase separators that split the segmented flow into two separate, homogeneous streams of liquid, and then inject the sample-containing stream into the ESI source. See, e.g., Zhu et al., Integrated Droplet Analysis System with Electrospray Ionization-Mass Spectrometry Using a Hydrophilic Tongue-Based Droplet Extraction Interface, Anal. Chem., 82, 8361-8366(2010). However, the phase separation approach may result in dilution and dispersion of the sample, which inhibit the detection capabilities of the ESI-MS system, as well as an excessive flow rate which may reduce ionization efficiency. Another approach has been to pump a segmented sample array directly into the high-voltage tip of an electrospray needle, whereby the electrospray is produced directly from the sample plugs. See, e.g., Pei et al., Rapid Analysis of Samples Stored as Individual Plugs in a Capillary by Electrospray Ionization Mass Spectrometry, Anal. Chem., 81, 6558-6561(2009); U.S. Pat. No. 8,431,888. In this latter approach, the segmented sample array consist of aqueous sample plugs coated with a small amount of oil and separated from each other by air. To be effective, this approach may be limited to the use of air as the isolating medium between sample plugs in order to avoid spraying oil into the ionization chamber. If liquids are used as the isolation medium, maintaining a stable spray when switching from the aqueous segment to the isolation liquid is likely to cause the spray to stop and the liquid exiting from the tip to remain on the tip and inhibit further spray. Moreover, this approach is limited solely to the use of ESI-MS as the detection technique.
There is a need for generating a stream of sample droplets from a segmented fluid flow that does not require or rely directly on the mechanism of electrospray. The sample droplet stream could thereafter be converted into an electrospray if desired, or could serve as the sample source for various other ionization techniques. More generally, the sample droplet stream could be utilized in a wide range of analytical techniques, including those not requiring ionization of the sample material.